This invention relates primarily to microlithography, and more specifically to extreme ultraviolet (EUV) lithography. The invention is applicable to semiconductor lithography and lithographic patterning of microstructures such as micromechanical systems, micro-optics, porous membranes, etc.
EUV lithography is a leading candidate technology for next-generation semiconductor lithography, which will require sub-100-mn printing resolution. An industry consortium (the EUV Limited Liability Company) is focusing development efforts on an engineering approach that is basically an evolutionary extension of LW and DUV projection lithography, the main difference being that the EUV system requires all-reflective optics due to the lack of EUV-transmitting lens materials (Ref. 1). The basic system design has nine reflective surfaces between the EUV source and the wafer, including four condenser mirrors, the photomask, and four projection camera mirrors. Seven of the reflectors (including the photomask) comprise multilayer coatings that operate at near-normal incidence and are optimized for peak reflectance at a wavelength of 13.4 nm.
Mirror coatings are one of the key enabling technologies that have made EUV lithography possible. Presently, multilayer Mo/Si (molybdenum/silicon) multilayer mirrors routinely achieve peak reflectance efficiencies of 67.5% at the 13.4-nm operating wavelength, with a spectral reflectance bandwidth of 0.56-nm FWHM (Refs. 2, 3). Nevertheless, the lithography system""s optical efficiency is significantly limited by the compound reflectance losses of seven multilayer mirrors operating in series.
The mirror surface figure tolerances scale in proportion to the operating wavelength, which is an order of magnitude smaller than DUV wavelengths at the 13.4-nm EUV wavelength. Furthermore, tolerance requirements for mirrors are generally much more stringent than lenses. Thus, the four EUV projection camera mirrors which image the mask onto the wafer must be fabricated to extremely tight (i.e., atomic-scale) absolute figure tolerances over large aperture areas.
The system uses multilayer reflective masks, which operate at 4:1 reduction. Mask fabrication presents significant technical challenges due to factors such as the short operating wavelength and small feature dimensions, lack of EUV pellicle materials, and very stringent registration requirements. EUV masks are expected to cost approximately $50,000 at 100-nm print resolution (Ref. 4).
Two research groups at the University of California at Berkeley and Stanford University are currently investigating maskless EUV lithography approaches (Refs. 5, 6). Their basic approach is to replace the mask with a spatial light modulator (SLM) comprising a programmable array of micromirrors whose reflectance properties can be varied by modulating the mirrors"" tilt or translational positions. Although these are maskless systems, they nevertheless require EUV projection optics to image the micromirror array onto the wafer. Due to minimum size limits of the SLM pixel elements, the projection optics must operate at a very high reduction ratio, (e.g., 20 to 40xc3x97 demagnification), which precludes the use of current-generation (4xc3x97) EUV projection optics.
Another limitation of micromirror systems is that the SLM must operate at a very high frame rate (e.g., multi-megahertz) in order to achieve requisite throughput requirements. (The system""s total data throughput requirement is at least 10 bits per second.) This requires a continuous EUV source such as a synchrotron, and precludes the use of a pulsed EUV source such as a high-power, laser-produced plasma (LPP) source that is currently being developed by Sandia National Laboratories for EUV lithography (Refs. 7, 8).
Several recent patents and publications describe a variety of other SLM-based maskless lithography design concepts, some of which are applicable to EUV (Refs. 9-18). Most of these systems employ projection optics, but one of these inventions, a Fresnel zone plate microlens system (Refs. 16-18), eliminates the need for projection optics as well as the mask. The system is designed primarily for x-ray lithography at a wavelength of 4.5 nm, but the basic design principle could be applied equally well to EUV. The device comprises an array of zone plate microlenses that focus an incident x-ray (or alternatively, EUV) beam onto an array of diffraction-limited points on a wafer surface. Each microlens transmission is modulated by a micro-actuated shutter as the wafer is raster-scanned across the focal point array to build up a synthesized, high-resolution image.
Since the zone plate system does not use projection optics, it could conceivably use a very large number of zone plate lenses in multiple, large-area arrays, all operating in parallel. Based on printing throughput requirements, the required modulation frame rate would scale in inverse proportion to the number of lenses, so with a sufficiently large number of lenses the system could conceivably use an LPP EUV source operating at a moderate frame rate (e.g., 6 kHz, Ref. 8). However, the proposed method for manufacturing the zone plates lenses (spatial-phase-locked e-beam lithography) is not very practical for large-quantity production, and there are also other complicating factors that preclude the use of an LPP source with the zone plate system.
The plasma source in the LPP has a fairly large spatial extent (approximately 200 xcexcm diameter, Ref. 7), and since each focus point on the wafer is a diffraction-limited image of the source, optical resolution would be limited by the source""s geometric image size. Furthermore, the LPP would be used in conjunction with EUV mirrors that have a 2% combined spectral bandwidth, and the zone plate""s chromatic dispersion within this band could also significantly limit focus resolution. The source""s geometric image size on the wafer and the image""s chromatic spread should preferably both be significantly smaller than the diffraction-limited image of an ideal monochromatic point source. This condition could be achieved, in principle, by using very small microlenses with short focal lengths; but the zone plate lens elements must be sufficiently large to accommodate the shutter mechanism, data paths, and supporting framework, while also maintaining an acceptably high aperture fill factor. Due to this aperture size limitation, a practical zone plate system would probably perform poorly with an LPP, and a more costly synchrotron source would thus be required.
Another limitation of zone plate lithography is that the printing resolution and contrast can be significantly degraded by extraneous diffracted orders. Continuous-profile, refractive microlenses such as those shown in Ref. 13 would not have this limitation, but for EUV application such microlenses would be either too highly absorbing or too small to be of practical utility when used in the mode described in Ref. 13.
The present invention overcomes the limitations of prior-art microlens printing systems by using multiple microlenses in series to focus exposure illumination onto a printing surface such as a semiconductor wafer. In a preferred embodiment of the invention, the microlenses are used in pairs, each pair consisting of first- and second-stage microlenses, respectively designated as lens L1 and lens L2, wherein L1 focuses illuminating radiation onto L2, and L2 thence focuses the radiation to a focal point on the printing surface. Each such microlens pair forms a printer pixel, and multiple such pixels are combined to form a printhead, which focuses the radiation onto multiple focal points on the printing surface. A modulator mechanism modulates the focal points"" exposure intensity levels, and as the points are modulated, the printhead is scanned relative to the printing surface (either the surface or the printhead, or both, can be moved for the scanning) to build up a synthesized, high-resolution exposure image on the print surface.
In general, a printer pixel can comprise a sequence of two or more microlenses L1, L2, . . . LN (N being an integer greater than 1), wherein each lens Lm (1xe2x89xa6m less than N) focuses the illuminating radiation onto the next lens Lm+1 in the sequence, and the last lens LN focuses the radiation onto a focal point on the printing surface. Typically L1 is a comparatively large element with low optical power, and the succeeding lenses are progressively smaller and have progressively higher optical power.
The modulator mechanism preferably comprises modulator elements that are incorporated as components of the printer pixels, with each modulator element providing independent, digital control of the exposure level at a corresponding focal point. However, in some embodiments a single modulator could be used to modulate the radiation before it is conveyed to the printhead. This option would be useful for applications that do not require independently modulated pixels, for example, in the fabrication of hole arrays for microfiltration membranes. The preferred embodiments use binary-state (i.e., ON/OFF) modulators, but gray scale (continuous-level) modulation may also be employed.
The disclosed embodiments relate primarily to the ETV lithography application, although the disclosed design methods and configurations are also applicable or adaptable to other wavelength ranges such as DUV. In a preferred embodiment, molybdenum microlenses are used to focus an incident EUV beam (at a wavelength of 11.3 nm) onto an array of diffraction-limited (58-nm FWHM) focal points on a wafer surface. A modulator mechanism, such as an SLM comprising microshutters proximate to the microlens apertures, modulates the beam intensity at each focal point. As the points are modulated, the focal point array is raster-scanned across the wafer to build up a digitally synthesized, high-resolution image.
The microlenses are preferably continuous-profile, refractive elements. A refractive EUV lens with sufficient optical power to achieve the requisite focus spot resolution ( less than 100-nm spot size) would have to be very smallxe2x80x94on the order of 1 xcexcm diameter or less; otherwise the lens would be too thick to allow acceptable EUV transmittance. A single array of microlenses and modulators of this size could not be practicably manufactured without incurring unacceptable fill factor losses. But this limitation is circumvented by using a multi-stage microlens system. In a two-stage embodiment, comparatively large (5-xcexcm diameter), low-power (NA=0.015) first-stage lenses focus EUV illumination onto much smaller (1-xcexcm diameter) second-stage lenses which have much higher optical power (NAxe2x89xa70.1). Each first-stage lens""s diffraction-limited focused spot overfills the 1-xcexcm aperture of a corresponding second-stage lens; and the second-stage lens focuses the illumination down to a much smaller (58-nm FWHM) focus spot on the wafer. This design approach makes it possible to use a sparsely distributed array of second-stage lens elements without incurring significant fill factor losses. The sparse distribution simplifies fabrication processes, and the wide spacing between second-stage lens apertures provides accommodation for structural support and for the SLM data paths and actuators.
The disclosed two-stage embodiments of the invention have a number of advantages over prior-art EUV lithography systems, including some or all of the following: (1) The need for EUV photomasks is eliminated. (2) By integrating the SLM with the microlens arrays, the need for image projection optics is eliminated. (3) The optical tolerance requirements for the microlenses are very moderate compared to all-reflective EUV projection optics. (4) Proximate image points are exposed sequentiallyxe2x80x94not simultaneouslyxe2x80x94so they do not interact coherently, and the printed image would thus be devoid of the kind of coherent proximity effects that are exhibited by projection systems. (5) The second-stage lens apertures function as spatial filters that eliminate stray radiation and scatter from sources such as flare in the EUV illumination optics. (6) The EUV illumination source is imaged onto the second-stage microlens apertures, not onto the wafer; so the source size and spatial energy distribution do not significantly affect optical resolution (although these factors do have a minor influence on optical efficiency). (7) In contrast to zone plate lenses, the refractive lens elements could produce very clean and highly resolved focus spots free of spurious diffraction orders and scatter, and exhibiting negligible chromatic dispersion.
The wafer exposure process is commonly referred to as xe2x80x9cprintingxe2x80x9d, and the combination of each associated first- and second-stage (or more generally, first- through N-th stage) microlenses and their associated modulator component is referred to herein as a xe2x80x9cprinter pixelxe2x80x9d. (In some alternative embodiments, the SLM need not be integrated with the microlens arrays and the focus spot intensities need not be controlled by separate modulator elements. In this case the pixels would not include the modulator components.) The pixels are assembled into arrays termed xe2x80x9cprintheadsxe2x80x9d; and the printheads are typically assembled into larger arrays, termed xe2x80x9cwafer print modulesxe2x80x9d, each of which covers and exposes a full 300-mm wafer. The complete exposure system (or xe2x80x9cprinterxe2x80x9d) typically comprises multiple wafer print modules covering separate wafers, which are all supplied EUV illumination from a single EUV source.
In a specific embodiment, each printhead comprises approximately 1.3xc3x97107 pixels distributed over a 20-mm square area. In addition to housing the microlenses and SLM components, each printhead can be equipped with peripheral microsensors that sense its position relative to an alignment pattern on the wafer, such as a periodic tracking pattern formed in the wafer scribe lines. Each printhead can also be provided with its own micro-positioning actuators that dynamically maintain focus, tilt, and overlay alignment in response to the position sensor feedback from a large number of microsensors. This design approach has two advantages relative to more conventional xe2x80x9cthrough the lensxe2x80x9d or xe2x80x9coff axisxe2x80x9d alignment sensors employed in projection lithography systems (Ref. 19, Chap. 5). First, the effect of nonsystematic errors such as noise in the tracking signals is minimized by using a large number of microsensors in the alignment tracking system. Second, by combining the printing microlenses, positioning sensors, and positioning actuators in a single, compact unit in close proximity to the wafer, tolerance stack-up in the tracking servo is minimized and very accurate and responsive tracking control can be achieved.
Thermal expansion differences between the printhead and the wafer could result in significant effective magnification errors and overlay misregistration. For example, a 0.1xc2x0 C. temperature difference could induce a 5-nm overlay error (which is most of the alignment tolerance budget). Magnification errors can be substantially eliminated by forming each second-stage microlens on an individually controllable micromechanical actuator which makes nanometer-scale lateral positioning adjustments in response to the tracking sensor feedback. The microlens actuators could also be used to correct a variety of other critical error factors. (For example, the allowable tolerance range on tilt and coma in the second-stage microlenses can be more than doubled by applying a calibrated position offset to each actuator.)
The system is designed for use with a high-power (1700 W), xenon LPP source operating at a 6 kHz repetition rate (Ref. 8). The system""s total data throughput requirement ( greater than 1012 bits per sec) and the low (6 kHz) frame rate necessitate the use of a very large number of printer pixels ( greater than 108). Furthermore, an even higher number of pixels is needed if the system is designed to optimize optical efficiency. The microlens efficiency is affected by the source collimation, and efficiency would be significantly compromised if the EUV illumination""s angular spread were much more than 1 mrad. Given that the source diameter is close to 200 xcexcm (Ref. 7), this implies that the collimation optics should preferably have a minimum effective focal length of at least 200 mm (i.e., 200 xcexcm/0.001 rad). With a focal length this large, acceptable optical efficiency can only be achieved by collecting radiation over a very large area; and hence a very large number of microlenses is used to efficiently utilize the source.
The high microlens number requirement is achieved by distributing the ETV illumination over a large number of printheads covering multiple wafers. The printer comprises a total of 360 printheads (i.e., 4.6xc3x97109 pixels), which are divided among eight wafer print modules, each module comprising 45 printheads distributed over a 300-mm wafer. Eight wafers are simultaneously exposed with a single LPP illumination source.
This xe2x80x9cmassively parallelxe2x80x9d design approach is feasible because the SLM is not imaged through projection optics. Aside from considerations of LPP source compatibility, this approach has the advantage that the SLM frame rate and tracking speed, which scale in inverse proportion to the number of pixels, would be very moderate. (The micromechanical and tracking servo design requirements for a 6 kHz system would be much simpler than alternative multi-megahertz SLM systems.) The SLM power consumption and radiative heat gain are also minimized with this approach. Additionally, the large number of printheads (360) makes the data flow easier to handle. The system""s total data rate for pattern generation is 2.8xc3x971013 bits per sec. This translates to 7.7xc3x971010 bits per sec per printhead (i.e., 2.8xc3x971013/360), which can be managed, for example, with several fiber optic data lines and integrated demultiplexers. The data storage and data processing hardware would make up a significant portion of the system cost, but this cost can be minimized by supplying a common data stream to all eight modules.
The EUI illumination encounters four mirror surfaces between the source and the printheads. These include a deep, aspheric condenser element, a shallow, spherical collimator element, and two sets of flat, terraced fold mirrors that partition the collimated beam into individual rectangular illumination fields on the printheads. The condenser and collimator mirrors are multilayer Mo/Be (molybdenum/beryllium) mirrors optimized for peak reflectivity at a wavelength of 11.3 nm, whereas the fold mirrors operate at glancing incidence and can therefore use a much simpler broadband reflective coating (such as a ruthenium film).
The Mo/Be mirrors have a reflectance bandwidth of 0.27-nm FWHM per mirror, about half that of Mo/Si mirrors optimized for 13.4 nm (Ref. 2). Nevertheless, the total reflected EUV power is much higher with Mo/Be mirrors because the xenon LPP source""s emission spectrum has a sharp peak near 11 nm (Ref. 7). The narrow bandwidth of Mo/Be mirrors is problematic with EUV projection systems because it is difficult to accurately match spectral reflectance peaks between seven mirrors (Ref. 3). But with just two multilayer mirrors the maskless system""s matching tolerance is much less critical, and it is easier to take advantage of the higher source emission at 11 nm.
The system""s printing resolution is estimated at 70 nm for mixed positive- and negative-tone images. Printing throughput is estimated at 62 300-mm wafers per hour, based on an assumed resist solubility threshold of 20 mJ/cm2. (The maximum attainable flood-exposure level is 80 mJ/cm2.) Projection EUV system designs typically assume a more stringent exposure sensitivity of 10 mJ/cm2, based on 100-nm print resolution and a 13.5-nm exposure wavelength. But the microlens system is designed for higher resolution (70-nm feature size) and operates at a shorter (11.3-nm) exposure wavelength, making the print quality more susceptible to line edge roughness induced by shot noise (Ref. 20). The higher 20-mJ/cm2 exposure threshold would counterbalance the system""s higher shot noise sensitivity. (If a 10-mJ/cm2 exposure threshold were assumed, throughput could be doubled.)
The microlens elements can be manufactured with nanometer-scale profile control by adapting shadow-mask deposition and etching processes that are used, for example, in shadow-mask molecular beam epitaxy (MBE), Refs. 21-24. Interference lithography methods can also be used to scale up the shadow-mask processes for high-volume production. Since the refractive index of molybdenum is very close to one at EUV wavelengths, the optical surface tolerances are comparatively loose in relation to mirror optics. The estimated figure tolerances are 4 nm RMS for the first-stage (5-xcexcm diameter) element and 2 nm RMS for the second-stage (1-xcexcm diameter) element, assuming that a calibrated lateral position offset is applied to the second-stage elements. (By comparison, EUV projection mirrors are required to meet a 0.25-nm RMS figure tolerance across aperture dimensions exceeding 100 mm, Ref. 1.) Additionally, the microlens system is very tolerant of isolated pixel defects. For example, an isolated nonfunctional pixel (i.e., a pixel that is permanently stuck in either the ON or OFF state) would only induce an estimated 1% uncorrected dimension shift on a 70-nm feature.